Dielectric loss measuring apparatus employing plural tuned circuits

ABSTRACT

An apparatus for the precise measurement of very low dielectric losses of a dielectric specimen at high frequencies having a high frequency oscillator, a series tuning circuit electromagnetically coupled to the coil in a tank circuit of the high frequency oscillator and having two series-connected variable air capacitors, two tuning circuits electrostatically coupled to the series tuning circuit, and a detecting circuit having an indicating meter and inductively coupled to the parallel tuning circuits. A dielectric specimen is connected to one of the parallel tuning circuits and the zero method is utilized to measure the loss resistance of the specimen.

0 United States Patent 1 3,585,496

[72] Inventor Buniiro lchllo [56] References Cited P UNITED STATES PATENTS 2,329,098 9/1943 Browning et a] 324/57 x (221 1969 2 906 950 9/1959 lchijo 324/61 x .[45] Patented June I5, 1971 [73] Assignee Tsuglmi Mfg. Co., Ltd. Primary Examiner-Rudolph V. Rohnec T k o, Ja n Assistant Examiner- Ernest F. Karlsen [32] Priority Jan. 5, 1968 Attorney-Hall, Pollock & Vande Sande [33] Japan 1 /4 ABSTRACT: An apparatus for the precise measurement of very low dielectric losses of a dielectric specimen at high [54] frequencies having a high frequency oscillator, a series tuning 8 cm 8 D in n circuit electromagnetically coupled to the coil in a tank circuit r. a of the high frequency oscillator and having two series-con- [52] US. Cl. 324/60, nected variable air capacitors, two tuning circuits electrostati- 324/57 cally coupled to the series tuning circuit, and a detecting cir- [5 1] Int. Cl Glnr 2 7 2 6, cuit having an indicating meter and inductively coupled to the G0lr 27/00 parallel tuning circuits. A dielectric specimen is connected to [50] Field of Search 324/60, 6|, one of the parallel tuning circuits and the zero method is util- 5 7,. 60 B, 60 C, 60 CD ized to measure the loss resistance of the specimen.

2 (12) P AAAV B Z i I f I L 2 AMPL/F/El? fb 6/ -\io o/ M2 '6} [mi 0501 e LA7'0/? 8 L 9. C02 L E AMPL/HEP BUNJlRp m INVENTUR BYMufM g UMLQJ ATTORNEY PATENTED JUN! 5 l9?! SHEET 2 [1f 2 FIG? 4a FIG. 4b

B M/Ra Ig INVENTOR PYR IVMJ ATTORNI-L'Y DIELECTRIC LOSS MEASURING APPARATUS EMPLOYING PLURAL TUNED CIRCUITS This invention relates to electrical measuring apparatus and more particularly to an apparatus for the precise measurement of electrical losses of dielectric materials.

Precise measurement of very low resistances, very high resistances and very low dielectric losses of various dielectric l materials at high frequencies is increasingly required with the advance in dielectric materials for use in electrical engineermg.

It is a primary object of the present invention to provide a novel and improved apparatus which satisfies the above demand.

The above and other objects, features and advantages of the present invention will be apparent from the following description when read in conjunction with the accompanying drawings, in which:

FIG. 1 is a circuit diagram of the electrical measuring apparatus of the present invention which is adapted to measure a very low dielectric loss at high frequencies by means of an electrostatic coupling system;

FIG. 2 is a circuit diagram showing details of part of the circuit diagram shown in FIG. 1;

FIGS. 30, 3b and 3c are vector diagrams of voltages appearingin the detecting circuit;

FIG. 4a is a circuit diagram of another embodiment of the present invention which is adapted to measure a very low dielectric loss at high frequencies by means of an inductive coupling system; and

FIGS. 4b and 4c are equivalent circuit diagrams of the loss measuring section in the embodiment shown in FIG. 4a.

Referring to FIG. 1, a high frequency oscillator OSC is stabilized in its high frequency output and is associated with a tank circuit including a capacitance C, and an inductance L,,. A coil having an inductance L, is inductively coupled to the inductance L with a coefficient of magnetic induction M The inductance L, and capacitances C, and C, form a series tuning circuit so as to measure an equivalent parallel resistance R, of a specimen from a minute change AC, in C, or a minute change AC in C The inductance L has a resist ance n, and the relation therebetween can be expressed as wL,/r0=Q,. Suppose now that C ,=C then the circuit comprising L,, C, and C is tuned when the relation co L,C,=2 (1) holds. and the voltage appearing across C, becomes maxim u m yariable air capacitors having the capacitances C, and C have their rotor sides grounded in common an d a 'e electrostatically coupled to parallel tuning circuits I and II at their high voltage terminals through very sm all capacitances C 0 and C of equal value, respectively. Coils having inductances L and L in the tuning circuits I and II are electromagnetically coupled to coils having inductances I, and I, in a detecting circuit with coefficients of mutual induction M 2 and M of equal value, respectively.

These two tuning circuits I and II are identical in their construction and circuit constants. Thus, L =L R =R and C =C When the series tuning circuit is tuned under the condition of C ,=C terminal voltages e, and e, are of the same magnitude or e,=e,'. Because of the fact that C =C maximum voltages appear across the two tuning circuits I and II when the following conditions are satisfied:

In this case, e =e Therefore, voltages e and e, in-

duced across the inductances I, and 1 are of the same magnitude or e,=er. and the deflection of an indicating meter 1,, is zero.

In the detecting circuit, a combination of a capacitance C an inductance L and a capacitance C is provided in order to amplify a minute difference Ae, between e, and e the minute difference being then amplified by an amplifier A for deflecting the indicating meter 1,, accordingly. In the state in which the circuits are in complete equilibrium and I,,=O, the inputs e, and e, to the detecting circuit have a vector relationship as shown in FIG. 3a. Then when a switch S is closed to connect a dielectric specimen (having a capacitance C; and equivalent parallel resistance R,) to the circuit I, e will now be changed into e in FIG. 3b due to the presence of C and R and an output AeI, appears across a terminal T, and the ground E to cause a deflection of the indicating meter I Then, the value of C may be reduced to minimize the deflection of the meter I g. In this case, e, will have a vector as shown in FIG. 3c and the value of C, can be obtained from the value of C The deflection of the indicating meter 1 in such a case is caused by Ae which is dependent upon the resistance loss R, of the specimen.

Referring next to FIG. 2, a method of seeking R, from C, will be described. In the equivalent circuit of the highfrequency oscillator, the high-frequency source generating a high-frequency voltage e, comprises a vacuum tube circuit having an internal resistance r,,. The relation between the internal resistance r,, and an equivalent parallel resistance R0 of the tank circuit including L and C is R0 r Now, the voltage 2 appearing across the tuning circuit I under the conditions of and will be sought in FIG. 2. Under the conditions that the circuits l and II are coupled to the series tuning circuit including L,, C, and C, through C and C0, and under the condition that the equation (5) is satisfied, series resistances r, and r, in the series tuning circuit can be given by the following equations:

(provided that wC,r, 1) and from /e /=wC' R e,, e is given by The outputs or sensitivity losses due to a minute change of R are given by Suppose that As is the difference between the voltages across the circuits I and 11, then Ae=Ae,e Because of the fact that C,,==C,,', C, and R =R Ae, is given i y as; R2

Voltages e, and c appearing as a result of minute change AR; in R or R R AR, will now be sought The actual output is the difference between 2 and e, in the equations and (ll') and acts to deflect the indicating meter. Suppose that C, is increased by a minute amount AC, so that the deflection becomes zero again. The voltages e and e, when C, is increased to C,+AC, are obtained from the equations (10) and (II), and the difference therebetween is assumed to be zero or e e,'=0. Then, the denominators of the equations (10') and l l) are entirely identical with each other and are given by which may be expressed herein as A. Thus,

Since C C C,= C, and R =R the above equation can be expressed as MQLORQQB g MQLQRZEB a A c, c, A

ARZ a z L m1 E c. A a cr The above equation can be simplified as AQJJE because the third member of the equation is negligible compared with the first and second members. On the other hand, the relation & & R R, holds when R R and finally the following equation is obtained:

fi &

C, R, (14) Since C, and R in the equation 14) are known values, R, can be obtained from the value of a minute change AC, in C, when the output due to the presence of R is made zero by varying the value of C,.

In another embodiment shown in FIG. 4a, the apparatus comprises a power supply regulating section and a measuring section which are electromagnetically coupled to each other. The power supply regulating section includes variable air capacitors having capacitances C, and C,. Coils having inductances L and L, are connected in parallel with the respective capacitors. The coils have respective resistances r, and r,'. The magnitudes of the inductances L, and L,f are selected so as to satisfy the conditions w'--L,C, 1 and w L,'c, 1, respectively. Supposeithat C 1=C C,'= CF," and C =C when the circuit including L 'C, and C,' is

tuned with a power source frequency f inthe state in which L, and L, are not connected. In this case, voltages appearing across C, and C, are maximum under the condition Then when the inductances L and L, are connected, the circuits are tuned under the conditions supply regulating section is given by (HI/1 Tu Z: (w L,C, 1 TL (FLCPDZ (16) wherein 4 (afiL,C, 1) Neglecting the first member under the root symbol since it is negligible compared with the second member, Z can be expressed as .VZ will L.

w L C 1 From the Equation 17), the following equation can be across the terminals T C =C C Because of the fact that r: is varied to r +Ar an output is developed to deflect the output indicating meter. The capacitance C is very slightly reduced by an amount AC, to reduce the output to zero again. Then, from the equations (20) and (21 we obtain usie-ne V l .7 W r2=ra2 because both the circuits are completely similar to each other, we obtain the following equation:

hence,

AQ 2 2) o r 0 2 0 2 Since C 01 $0.01 and we may neglect the third member in the parentheses in 65 the above equation and obtain the following equation: -A2

Thus, Ar hence r, can be sought from the equation (22) 70 because C r, and AC are known. It will be understood that r, is expressed as Since AC, can precisely be measured from a change in indication in the dial graduations of the standard variable air capacitor, the value of Ar can accurately be measured Alternatively. the output due to Ar may be made zero by changing C0 to (,,AC,' and to +AC, In this case, from the equations (20) and (21 we obtain More precisely, the relation AC =%AC1 is obtained in'; this case. This second method is featured by the fact that the variation in the tuned state of the series tuning circuit due to AC 1 is less than in the first method.

From the foregoing description, it willbe understood that the present invention employs the zero method in which precisely variable air capacitors are manipulated for the precise measurement of resistance, and in the detecting section, a minute difference voltage is solely amplified for deflecting an indicating meter whereby to measure a very small loss.

What I claim is:

1. An apparatus for the precise measurement of very low dielectric losses of a dielectric specimen at high frequencies comprising:

a stabilized high frequency oscillator;

a tank circuit connected to said oscillator;

a resonant circuit electromagnetically coupled to said tank circuit, and having an inductor and two variable air capacitors of equal capacity all connected in series, said capacitors being grounded at their common connected ends;

first and second parallel tuned circuits having the same construction and circuit constants each having one end grounded and each being electrostatically coupled to said resonant circuit at the opposite ends of said grounded capacitors through very small capacitances of equal value;

a secondary detecting circuit coupled to said first and second parallel tuned circuits;

indicator means responsive to the vectorial addition of voltages of opposite phase in said detecting circuit for providing a manifestation of the resonance of said first and second tuned circuits;

and circuit means for connecting said specimen in parallel with said first parallel tuned circuit.

2. The apparatus according to claim 1, further comprising 1 means for differentially varying the capacitances of resonant circuit.

3. The apparatus according to claim 1, in which said first and second parallel tuned circuits include respective variable capacitors grounded at their one ends.

4. The apparatus according to claim 1, in which said variable air capacitors are grounded at their rotors.

5. An apparatus for the precise measurement of very low 7 dielectric losses of a dielectric specimen at high frequencies comprising:

a stabilized high frequency oscillator;

a tank circuit connected to said oscillator;

a resonant circuit having an inductor coupled to the coil of said tank circuit and first and second variable capacitors all connected in series, said first and second capacitors being grounded at their one ends and being of equal capacity;

first and second coils having the same construction and circuit constants connected in parallel with said first and second capacitors, the capacitances C and C, of said first and second capacitors and the inductances L, and and circuit means having test electrodes coupled to said first L, of said first and second coils having the relations of tuned circuit for connecting one specimen in parallel with w LiCi l and w L .'(l e l. where n) represents angle h m frequency. with said first and second variable capacitors r y the I055 and Capacitance Components of the being adjusted o maintain said serie tun ng i i at 5 specimen under test are derivable from a minute change resonance; in at least one of the series-connected capacitors when first and second tuned circuits each having the same conthe output of said dectecting input due to the presence of struction and circuit constants, said first and second the specimen is made zero.

tuned circuits having third and fourth coils respectively. 6. The apparatus according to claim 5 which further comsaid third and fourth coils being electromagnetically coul prises means for differentially varying the capacitances of said pled to said first and second coils with the same coupling resonant i it,

P Y B 7. The apparatus according to claim 5 in which said first and an mdcatmg mducm,electromagnuclly second parallel tuning circuits includes respective variable coupled to said third and fourth coils and an indicating capacitors grounded at their one ends meter whose deflection IS set at zero under normal condi- 8. The apparatus according to claim 5 in which said variable trons so that the output voltages from said first and air capacitors are grounded at their rotors second circuits in their tuned state are equal to each other; 

1. An apparatus for the precise measurement of very low dielectric losses of a dielectric specimen at high frequencies comprising: a stabilized high frequency oscillator; a tank circuit connected to said oscillator; a resonant circuit electromagnetically coupled to said tank circuit, and having an inductor and two variable air capacitors of equal capacity all connected in series, said capacitors being grounded at their common connected ends; first and second parallel tuned circuits having the same construction and circuit constants each having one end grounded and each being electrostatically coupled to said resonant circuit at the opposite ends of said grounded capacitors through very small capacitances of equal value; a secondary detecting circuit coupled to said first and second parallel tuned circuits; indicator means responsive to the vectorial addition of voltages of opposite phase in said detecting circuit for providing a manifestation of the resonance of said first and second tuned circuits; and circuit means for connecting said specimen in parallel wiTh said first parallel tuned circuit.
 2. The apparatus according to claim 1, further comprising means for differentially varying the capacitances of resonant circuit.
 3. The apparatus according to claim 1, in which said first and second parallel tuned circuits include respective variable capacitors grounded at their one ends.
 4. The apparatus according to claim 1, in which said variable air capacitors are grounded at their rotors.
 5. An apparatus for the precise measurement of very low dielectric losses of a dielectric specimen at high frequencies comprising: a stabilized high frequency oscillator; a tank circuit connected to said oscillator; a resonant circuit having an inductor coupled to the coil of said tank circuit and first and second variable capacitors all connected in series, said first and second capacitors being grounded at their one ends and being of equal capacity; first and second coils having the same construction and circuit constants connected in parallel with said first and second capacitors, the capacitances C1 and C1'' of said first and second capacitors and the inductances L1 and L1'' of said first and second coils having the relations of omega 2L1C1> 1 and omega 2L1''C1'' > 1, where omega represents angle frequency, with said first and second variable capacitors being adjusted to maintain said series tuning circuit at resonance; first and second tuned circuits each having the same construction and circuit constants, said first and second tuned circuits having third and fourth coils respectively, said third and fourth coils being electromagnetically coupled to said first and second coils with the same coupling respectively; an indicating circuit having inductors electromagnetically coupled to said third and fourth coils and an indicating meter whose deflection is set at zero under normal conditions so that the output voltages from said first and second circuits in their tuned state are equal to each other; and circuit means having test electrodes coupled to said first tuned circuit for connecting one specimen in parallel with the former; whereby the loss and capacitance components of the specimen under test are derivable from a minute change in at least one of the series-connected capacitors when the output of said dectecting input due to the presence of the specimen is made zero.
 6. The apparatus according to claim 5 which further comprises means for differentially varying the capacitances of said resonant circuit.
 7. The apparatus according to claim 5 in which said first and second parallel tuning circuits includes respective variable capacitors grounded at their one ends.
 8. The apparatus according to claim 5 in which said variable air capacitors are grounded at their rotors. 